feat(zh): add Chinese translations in zh/ directory

Add Chinese (Simplified) translations for all documentation, organized
under a dedicated zh/ directory that mirrors the English folder structure.

Co-authored-by: tanqingkuang <tanqingkuang@users.noreply.github.com>

Translations originally contributed by @tanqingkuang in #45.
Restructured from *-CN.md suffix pattern into zh/ directory to prevent
the EPUB builder (scripts/build_epub.py collect_folder_files) from
picking up Chinese files via glob("*.md") inside module folders.

zh/03-skills/doc-generator/SKILL.md

@@ -0,0 +1,76 @@
+---
+name: api-documentation-generator
+description: 从源代码生成全面且准确的 API 文档。适用于创建或更新 API 文档、生成 OpenAPI 规范，或在用户提到 API 文档、端点或说明时使用。
+---
+
+# API 文档生成 Skill
+
+## 可生成内容
+
+- OpenAPI / Swagger 规范
+- API 端点文档
+- SDK 使用示例
+- 集成指南
+- 错误码参考
+- 认证指南
+
+## 文档结构
+
+### 每个端点的写法
+
+```markdown
+## GET /api/v1/users/:id
+
+### 描述
+简要说明这个端点做什么
+
+### 参数
+
+| 名称 | 类型 | 必填 | 说明 |
+|------|------|------|------|
+| id | string | 是 | 用户 ID |
+
+### 响应
+
+**200 成功**
+```json
+{
+  "id": "usr_123",
+  "name": "John Doe",
+  "email": "john@example.com",
+  "created_at": "2025-01-15T10:30:00Z"
+}
+```
+
+**404 未找到**
+```json
+{
+  "error": "USER_NOT_FOUND",
+  "message": "User does not exist"
+}
+```
+
+### 示例
+
+**cURL**
+```bash
+curl -X GET "https://api.example.com/api/v1/users/usr_123" \
+  -H "Authorization: Bearer YOUR_TOKEN"
+```
+
+**JavaScript**
+```javascript
+const user = await fetch('/api/v1/users/usr_123', {
+  headers: { 'Authorization': 'Bearer token' }
+}).then(r => r.json());
+```
+
+**Python**
+```python
+response = requests.get(
+    'https://api.example.com/api/v1/users/usr_123',
+    headers={'Authorization': 'Bearer token'}
+)
+user = response.json()
+```
+```